The spinal nerve transection model of neuropathic pain is associated with alterations of the molecular organisation of the nodal regions of myelinated sensory axons within the dorsal root

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Abstract

Saltatory conduction in myelinated fibres depends on a specific molecular organization of axonal domains: the nodal, paranodal and juxtaparanodal regions. During development, shaker-type Kvchannels (STKC: Kv1.1 and Kv 1.2) at nodal and paranodal regions are essential to prevent hyperexcitation. At later stages, they cluster at juxtaparanodal regions and become ‘‘silent’’. Neuropathic pain (NP) develops following peripheral nerve injury and a critical event is the development of hyperexcitability in damaged myelinated sensory fibres. It
is known that at the site of injury glial and axonal architecture are disrupted. In this study, we aimed to investigate sensory fibres’ nodal architecture at a site remote from the nerve injury: the dorsal root (DR). L5 spinal nerve transection (SNT) was performed in rats and the L5 DR harvested at day 7 or 21 post injury. We used a pan-Nav channel antibody to label the node, Caspr and Neurofascin antibodies to label the paranode and a Kv1.2 antibody to label the juxtaparanode. Acutely after SNT the structure of the nodal region was preserved, but at a later time point we noted movement of Kv1.2 channels into the paranodal region such as there was co-labelling of Caspr and Kv1.2. There was a significant reduction in the longitudinal distance between the node and Kv1.2 immunostaining (p=0.04). There was no apparent change in nodal or paranodal architecture. Ongoing studies with electron microscopy will provide information about ultrastructural changes that may be associated to redistribution of Kv1.2. Quantification of juxtaparanodal protein expression after SNT will be done using Western blots. Redistribution of Kv1.2 after SNT might be a protective mechanism to counteract the nerve hyperexcitability that follows nerve injury. To test this hypothesis we are currently investigating the electrophysiological properties of the injured nerve fibres using an ex vivo DRG intracellular recording technique and probing the function of STKC by using the specific blocker α-dendrotoxin. Preliminary data shows that α-dendrotoxin significantly prolonged the action potential half width as well as refractory period in the injured but not in uninjured A-fibre neurons. Thismight suggest upon injury, STKC become actively involved in action potentials from a ‘‘hibernating’’ state in uninjured neurons.